Colour Change
Colour change phenomenon in gemstones including alexandrite, colour-change garnet, sapphire, and diaspore with causes and grading.
Introduction
Colour-change gems display a different dominant hue depending on the spectral
composition of the illuminant, most famously shifting between daylight and
incandescent light. The benchmark example is alexandrite (colour-change
chrysoberyl), described as "emerald by day, ruby by night": it appears green
to teal under daylight or fluorescent light (6500 K) and red to purplish-red
under incandescent tungsten (2700 K). [1]
The mechanism requires a dual transmission window in the gem's absorption
spectrum: the stone transmits both red (650–700 nm) and blue-green
(450–510 nm) wavelengths simultaneously. Which colour the eye perceives
as dominant is determined by the spectral power distribution of the light
source in those two regions. In alexandrite this is caused by Cr³⁺ in the
chrysoberyl crystal field; in colour-change garnets (pyrope-spessartine
composition) it involves both Cr³⁺ and V³⁺. [2]
The degree of change, from weak (<50%) to complete (100%), is the primary
quality criterion. [3]
Mechanism
How colour change occurs:
The Physical Cause
- Gem's absorption spectrum has transmission windows in both red and green
- Daylight is balanced across the spectrum → green component dominates
- Incandescent light is red-rich → red component dominates
- Eye perceives the dominant transmitted colour [2]
Requirements
- Specific absorption spectrum with dual transmission windows
- Usually involves chromium, vanadium, or iron chromophores
- Balance between absorbed and transmitted wavelengths
- Light source spectral composition must differ significantly
Alexandrite
Alexandrite is colour-change chrysoberyl, the most famous and valuable colour-change gem.
The Colour Change
- Daylight/fluorescent: Green to blue-green
- Incandescent: Red to purple-red
- Cause: Chromium (Cr³⁺) absorption
- Ideal: Complete change from pure green to pure red
Grading Colour Change
| Percentage | Quality Description |
|---|---|
| 100% | Complete change (pure green ↔ pure red) |
| 75-99% | Strong colour change |
| 50-74% | Moderate colour change |
| <50% | Weak colour change |
Quality Factors
- Degree of change: More complete = more valuable
- Attractiveness: Both colours should be appealing
- Saturation: Vivid colours in both lights
- Clarity: Eye-clean preferred
- Size: Large stones very rare
Alexandrite Sources
| Origin | Daylight Colour | Incandescent Colour | Notes |
|---|---|---|---|
| Russia (Urals) | Green | Red | Classic; historic; depleted |
| Brazil | Blue-green | Purple-red | Major current source |
| Sri Lanka | Olive/yellowish | Brownish-red | Often less distinct change |
| Tanzania | Green | Red | Some fine material |
| India | Variable | Variable | Emerging source |
Other Colour-Change Gems
| Gem | Daylight | Incandescent | Cause |
|---|---|---|---|
| Colour-change sapphire | Blue/violet | Purple/pink | Vanadium |
| Colour-change garnet | Blue-green | Red-purple | Vanadium + chromium |
| Colour-change spinel | Blue | Violet | Cobalt or iron |
| Colour-change diaspore | Yellow-green | Pink-red | Manganese |
| Colour-change fluorite | Blue | Purple | Rare earth elements |
Colour-Change Garnet
Some of the most dramatic colour changes occur in garnet:
Characteristics
- Usually pyrope-spessartine composition
- Can show blue-green to red-purple change
- Some match alexandrite's change quality [2]
- Relatively rare
Sources
- Madagascar (best known)
- Tanzania
- Sri Lanka
- USA (Idaho)
Testing Light Sources
Market Considerations
Colour-change gems in the market:
- Alexandrite: Extremely valuable; strong demand
- CC sapphire: Premium over standard sapphire
- CC garnet: Significant value when strong change
- Synthetic: Synthetic colour-change gems exist (alexandrite, sapphire)
- Identification: Chemical testing may be needed for species confirmation
References
- ↑ 1. Read, P. (2014). Gemmology (3rd ed.). Butterworth-Heinemann/Routledge. DOI: 10.4324/9780080507224.
- ↑ 2. Qiu, Z.; Guo, Y. (2021). Explaining Colour Change in Pyrope-Spessartine Garnets. Minerals, 11(8), 865. DOI: 10.3390/min11080865.
- ↑ 3. Schumann, W. (2009). Gemstones of the World (4th ed.). Sterling. ISBN: 978-1-4027-6829-3.